EVOLUTION OF BREATHING HABITS 105 



history of every group that has attained supremacy. 

 The earlier acts are done under water. 



The simplest animals, so-called Protozoa, are 

 all characteristically aquatic. They either live freely 

 in water, or in a water-bath which some animal or 

 plant provides for them. Furnished with oars or 

 ' cilia ' they pursue and engulf their prey ; or, if station- 

 ary, draw in liquid instead of being drawn through it. 

 Thus both inside and outside they are bathed by 

 constantly changing water ; and from one or both 

 of these sources they extract the minimal quantity 

 of oxygen which is dissolved in such water, and it 

 suffices them. By its aid, movement is maintained, 

 the food broken down, and the living substance, after 

 rebuilding and replenishing itself out of that food, is 

 disintegrated as the first step in another cycle of life. 

 Neither lungs nor gills are distinguishable. There is 

 no blood to carry oxygen or to remove carbonic acid. 

 The protozoon is a gill and heart in effect, though 

 not in fact. It does with its seemingly simple tools 

 all that taxes the complicated structure of a higher 

 animal to accomplish in breathing, namely, to abstract 

 oxygen from the air dissolved in the water, to convey 

 that oxygen whither it is needed, to use the store 

 without exhausting it, and after use to carry away the 

 ashes and smoke of the fire. 



Sponges, corals, and anemones are animals whose 

 bodies are channelled out for the passage of water. 

 These creatures irrigate their bodies by a stream that 

 passes in through one or more openings, then through 



